1. Field of the Invention
The present invention relates to a liquid crystal driving signal generating apparatus for driving liquid crystal and an optical disk reproducing apparatus having the liquid crystal driving signal generating apparatus.
2. Description of the Related Art
In recent years, a digital video disk (DVD) has been proposed as an information recording (reproducing) medium with a large capacity exceeding a compact disk (hereinafter referred to as xe2x80x9cCDxe2x80x9d).
The DVD, which is an optical disk with the same diameter as that of the CD, that is, 12 cm, employs the wavelength of a laser light source of 650 nm which is shorter than that of 780 nm employed in the CD and further employs an objective lens with the numerical aperture of 0.6 which is larger than that of 0.45 employed in the CD.
Further, the DVD employs MPEG2 as the data compression algorithm.
Since the DVD has been improved in this manner as compared with the CD, it is possible to store digital data of about 5 giga byte on one side of a disk.
However, it has been said that a reproducing apparatus for reproducing a DVD, which is a high density medium much more than the CD, requires a control means for controlling the inclination, that is, the tilt angle of the disk surface relative to the optical axis of an optical pickup.
The applicant of the present application has already proposed in Japanese Patent Unexamined Application No. Hei 9-128785 that the aberration of the wave surface caused by the inclination of the disk surface and the variation of the disk thickness is corrected by using a liquid crystal panel.
To be more concrete, this proposal is characterized in that a liquid crystal panel for the aberration correction split into a predetermined configuration is disposed on the optical axis of a laser beam, whereby the aberration of the wave surface caused by the inclination of the disk surface and the variation of the disk thickness is corrected by controlling a refractive index of each of the split portions.
FIG. 8 shows an example of an optical disk reproducing apparatus using such a theory.
In FIG. 8, a laser beam radiated from a laser light source 1 is reflected by a half mirror 2.
The laser beam reflected by the half mirror 2 passes through a liquid crystal panel 3 and applied to an objective lens 4, which in turn forms a laser beam spot on an optical disk 5.
The reflection light reflected by the optical disk 5 passes again the objective lens 4, liquid crystal panel 3 and half mirror 2, and then converged on a photo receptor 7 by a condenser lens 6.
A tilt sensor 8 for detecting the inclination of the optical disk 5 is provided in adjacent to a pickup. The tilt sensor 8 is provided with one light emitting portion and two light receiving portions so that light irradiated from the light emitting portion on the optical disk 5 is reflected thereby and received as reflection light by the respective light receiving portions.
An adder 10 detects a difference between the outputs from the respective light receiving portions and supplies the difference as a tilt error signal to an analog-to-digital (A/D) converter 11.
The tilt error signal is converted into a digital value by the A/D converter 11 and supplied to a central processing unit (CPU) 17.
Upon occurrence of the tilt error, the CPU 17 outputs to a gain adjuster 13 a control signal for correcting the wave surface aberration caused by the tilt error.
An oscillation circuit 12 outputs a reference signal whose duty ratio is set to be 50(%) like a rectangular wave, for example.
The gain adjuster 13 adjusts an amplification degree of the reference signal supplied from the oscillation circuit 12 under the control of the CPU 17.
Thus, a driving signal inputted into the liquid crystal panel 3 is a signal with a duty ratio of 50% whose voltage amplitude is adjusted in accordance with the inclination of the disk.
The liquid crystal panel 3 is formed by sealing liquid crystal between two transparent glass substrates.
FIGS. 9A and 9B are diagrams showing an example of the configuration of the liquid crystal panel 3. FIG. 9B is a cross sectional view of the liquid crystal panel 3, and FIG. 9B is a plan view of the liquid crystal panel 3.
Transparent electrodes 302a, 302b such as indium tin oxide (ITO) electrodes are deposited on the inner surface of transparent glass substrates 301a, 301b. 
At least one of the transparent electrodes 302a, 302b has the electrode configuration split longitudinally and latitudinally into electrode portions as shown in FIG. 9B. Each of the electrode portions thus split is connected to corresponding one of the gain adjuster 13 so that voltages applied to the electrode portions are controlled under the control of the CPU 17.
Orientation films 303a, 303b for providing the liquid crystal with predetermined molecule orientation are formed on the inner surfaces of the transparent electrodes 302a, 302b. 
Liquid crystal 304 is sealed between the orientation films 303a, 303b. The liquid crystal 304 is liquid crystal having birefringence effect such that a refractive index (n1) of the liquid crystal molecule M to the optical axis direction differs from a refractive index (n2) thereof to the direction opposite to the optical axis direction. Thus, the liquid crystal may be nematic liquid crystal, for example.
FIGS. 11A to 11C are diagrams showing changes in the orientation of liquid crystal when voltage applied to the liquid crystal 304 is changed. FIG. 11A shows the orientation of the liquid crystal molecule M when the voltage applied to the liquid crystal is zero, while FIGS. 11B and 11C show the orientations of the liquid crystal molecule when alternative voltage is applied to the liquid crystal. In this case, the applied voltages are in a relation of e1 less than e2.
In this manner, the orientation of the liquid crystal molecule M can be controlled from the horizontal direction to the vertical direction by changing the alternative voltage applied thereto.
In FIGS. 11A to 11C, the respective portions of the liquid crystal 304 corresponding to all the split electrode portions are oriented to the same direction since the same control voltage is applied to the respective split electrode portions. However, it is possible to set suitable refractive indexes (n1-n2) for the respective portions of the liquid crystal 304 corresponding to the respective electrode portions by separately controlling the voltage applied to the respective electrode portions split longitudinally and latitudinally.
When the refractive index of the liquid crystal is changed, it is possible to provide a light beam passing through the liquid crystal with an optical path difference given by the following expression.
optical path difference=xcex94nxc3x97d
where xcex94n represents a changed value of the refractive index and d represents the thickness of the liquid crystal 304.
This means that the light beam passing through the liquid crystal is provided with a phase difference given by the following expression.
phase difference=xcex94nxc3x97dxc3x97(2xcfx80/xcex)
where xcex represents a wavelength of the light beam.
Accordingly, it is possible to correct the wave surface aberration caused by the inclination of the disk by controlling the refractive indexes n of the respective portions of the liquid crystal corresponding to the split electrode portions so as to cancel the aberration caused in the objective lens 4.
In the liquid crystal driving signal generating apparatus for controlling the voltage amplitude of the driving signal, there arises a problem that how the control of the voltage amplitude of the driving signal for driving liquid crystal is realized.
The present invention has been made in view of the above circumstances, and therefore an object of the present invention is to provide a novel liquid crystal driving signal generating apparatus.
Another object of the present invention is to provide an optical disk reproducing apparatus with such a liquid crystal driving signal generating apparatus.
In order to attain the above object, a liquid crystal driving signal generating apparatus according to the present invention comprises a PWM signal generator, a low pass filter for limiting a frequency band of a PWM signal outputted from the PWM signal generator, a liquid crystal panel connected to an output of the low pass filter, and a control means for controlling the PWM signal generator, and is characterized in that the control means controls the PWM signal generator such that the PWM signal generator generates a PWM signal with a duty ratio corresponding to a refractive index to be set in the liquid crystal panel.
Further, in order to attain the above object, an optical disk reproducing apparatus including a liquid crystal driving signal generating apparatus according to the present invention is characterized by comprising a PWM signal generator, a low pass filter, connected to the liquid crystal panel, for limiting a frequency band of a PWM signal outputted from the PWM signal generator, means for detecting a tilt angle of a disk, and a control means for determining a refractive index to be set in the liquid crystal panel for correcting wave surface aberration caused by the tilt angle, and for controlling the PWM signal generator such that the PWM signal generator generates a PWM signal with a duty ratio corresponding to the refractive index determined.
The above and other objects and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings.